To improve the transmission efficiency for digitized television video signals, a variable-length code technique has been used in prior arts such as variable-length differential pulse-code modulation systems and predictive encoding transmission systems based on interframe correlation. In either case, video signals to be transmitted are sampled at a sampling rate proportional to a predetermined scanning rate. However, since significant information to be transmitted is distributed at random with repsect to time, a buffer memory for temporarily storing encoded digital signals is needed on the transmitter side to transmit them at a predetermined bit rate. Correspondingly, another buffer memory for temporarily storing the digital signals transmitted is needed at the receiving end. Also, it is necessary to decode the received digital signals at the same sampling rate as that in the transmitter so as to avoid an overflow or underflow of the buffer memory at the receiver. In order to avoid such an overflow, the buffer memory of the receiver is required to have a capacity sufficiently larger than that of the transmitter.
To remove this restriction imposed on the receiver, a transmission system for digitized video signals has been proposed in the U.S. Pat. No. 4,027,100 issued May 31, 1977. According to this prior art, a buffer memory at the transmitter stores synchronization signals produced regularly, signals indicative of a buffer-occupancy state immediately following the respective synchronization signals, and information signals produced between each signal indicative of a buffer-occupancy state and the succeeding synchronization signal unevenly in response to the video signal to be transmitted. A buffer memory at a receiver stores the signals transmitted from the transmitter. A decoder coupled to the receiver-side buffer memory decodes the information signals at a decoding rate controlled with reference to the difference between an actual sum of buffer occupancies of both buffer memories and a value predetermined for the sum. However, this system can not be applied to the transmission whose transmission speed is constant.
Besides, a television video signal transmission system based on a time-division multiplex technique has been disclosed by Kaneko et al, in "Digital Transmission of Broadcast Television with Reduced Bit Rate", National Telecommunications, volume 3, 1977, section No. 41, pages 4-1 to 4-6. With this prior art, each channel information in one frame is assigned depending on the amount of information to be transmitted. However, because the transmission speed of information is not constant with respect to time, the improved technique disclosed in the U.S. Pat. No. 4,027,100 cannot be adapted to the second prior art.
An object of this invention is therefore to provide a digital transmission system for television video signals.